The present invention relates to valves utilized to control the application of braking pressure by an anti-lock braking system and in particular to a valve for use in such systems utilizing simple on-off control of braking pressure.
Low cost anti-lock braking systems have been developed wherein modulation of the braking force is effected by means of a single build/decay modulating valve for each channel of the anti-lock braking system. The single modulating valve is operable between two states to provide build and decay of braking pressure and such a modulating valve can be provided for operation by a single solenoid and a shuttle valve element responsive to a differential pressure between a wheel brake cyinder and a source of pressurized braking fluid. This type of system is commonly referred to as a "bang bang" system inasmuch as the transition between build and decay is relatively harsh and the system is typically not capable of operating in a pressure hold state. To minimize the harshness of the transition between build and decay modes, it has been a common practice to provide a flow controlling orifice in the valve which limits the rate of pressure build. Unfortunately the required orifice must be relatively small and as a result the orifice is subject to contamination. Because the orifice is typically incorporated into a shuttle valve element within the modulator valve, alteration of the orifice and correspondingly the build rate is not easily effected. Further, the orifice provided in the modulator valve can exhibit false shuttling of the shuttle valve element during normal braking, typically, during spike applications of the brakes.
There therefor exists a need for a valve that can be utilized with an anti-lock braking system which will obviate these disadvantages.
Broadly, the present invention is a valve assembly for use in an anti-lock braking system. The valve is provided with a first fluid passage having an inlet connected to a master cylinder and an outlet connected to a pressure responsive anti-lock braking system modulator valve. A second fluid passage having a flow control orifice therein has an inlet connected to source of pressurized brake fluid and an outlet connected to the modulator valve. A check valve is provided in the second passage for blocking fluid flow from the first passage to the pressurized source inlet. Valve means responsive to a differential pressure betwee the pressurized source and the master cylinder inlet is provided for closing the master cylinder inlet.
In specific embodiment of the invention, the valve means includes a piston operably connected between the master cylinder inlet and the pressurized source inlet to close the master cylinder inlet when the pump pressure exceeds the master cylinder pressure.
It is therefor an object of the invention to provide an orifice valve for use in anti-lock braking systems.
Another object of the invention is to provide such a valve which includes a flow control orifice which is operable in the system only during anti-lock braking operation.
Still another object of the invention is to provide such a valve in which the flow control orifice is readily changed.
Another object of the invention is to provide such a valve including a differential pressure responsive piston effective to connect a flow control orifice into the braking system during anti-lock braking operation.
Still another object of the invention is to provide such a valve in which a differential pressure piston provides positive isolation of the anti-lock braking system from the master cylinder.